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1.
The subsolidus phase equilibrium diagram for the pseudobinary join MgAl2 O4 -Ga2 O3 was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga2 O3 in MgAl2 O4 decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg2+ ions by Ga3+ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga2 O3 (=δ-Al2 O3 structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature. 相似文献
2.
V. S. STUBICAN C. GRESKOVICH H. A. McKINSTRY 《Journal of the American Ceramic Society》1969,52(4):174-177
A quantitative X-ray technique for measuring precipitation strains has not been previously applied in metallic or oxide systems. The Warren-Averbach analysis of strain was used to determine the buildup of elastic strain energy in the spinel crystalline solution matrix (gross composition = 60 mol% MgAl2 O2 + 40 mol% Cr2 O3 ) during the isothermal (1135°C) precipitation of a metastable (coherent) monoclinic phase. The elastic strain energy of the spinel crystalline solution matrix increased to a maximum of about 3.1 × 107 ergs/cm3 for a reaction time of 8 h. There was a marked decrease in the elastic strain energy during the initial precipitation of the equilibrium corundum crystalline solution with the composition (Al3+ 0.72 Cr3+ 0.25 )O3 . An overall diffusion activation energy for precipitation of the mono-clinic phase was approximately 86 kcal/mol. 相似文献
3.
Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al2 O3 at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl2 O4 –NiAl2 O4 solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl2 O4 from its oxide components (MnO + Al2 O3 ) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol−1 , respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models. 相似文献
4.
J. P. COUTURES 《Journal of the American Ceramic Society》1985,68(3):105-107
A tentative phase diagram for the system Al2 03 -Nd2 O3 is presented. Three compounds were obtained: a β -A12 O3 -type compound, the perovskite NdAlO3 , and Nd4 Al2 O9 . The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al2 O3 , mp 1900°C; Nd4 Al2 O9 , mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al2 O3 , 1750°C; 23 mol% Al2 O3 ,1800°C. Nd4 Al2 O9 decomposes in the solid state at 1780°C. 相似文献
5.
Liquidus equilibrium relations for the air isobaric section of the system Y2 O3 –Fe2 O3 –FeO–Al2 O3 are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y 0.22 Fe 1.08 Al 0.70 O 2.83 * Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y 0.75 Fe 0.65 Al 0.60 O 3 garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y 0.75 Fe 1.25 O 3 and Y 0.75 Fe 0.32- Al 0.93 O 3 can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior. 相似文献
6.
Bertha Alicia Vázquez Ángel Caballero Pilar Pena 《Journal of the American Ceramic Society》2005,88(7):1949-1957
Solid-state compatibility and melting relations of MgAl2 O4 in the quaternary system Al2 O3 –CaO–MgO–SiO2 were studied by firing and quenching selected samples located in the 65 wt% MgAl2 O4 , plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl2 O4 was constructed from CaO, SiO2 and exceeding Al2 O3 , not involved in stoichiometric MgAl2 O4 formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO2 , and Al2 O3 impurities on the high temperature behavior of spinel materials was also discussed. 相似文献
7.
J. J. COMER N. C. TOMBS J. F. FITZGERALD 《Journal of the American Ceramic Society》1966,49(5):237-240
Single-crystal and polycrystalline films of Mg-Al2 O4 and MgFe2 O4 were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al2 O8 film. Above 540°C, the γ-Al2 O, and MgO reacted to form a single-crystal MgAl2 O4 film with {001} MgAl2 O4 ‖{001} MgO. Above 590°C, an additional layer of MgAl2 O4 , which is polycrystalline, formed between the γ-Al2 O3 and the single-crystal spinel. Polycrystalline Mg-Al2 O4 formed only when diffusion of Mg2+ ions proceeded into the polycrystalline γ-Al2 O3 region. Corresponding results were obtained for Mg-Fe2 O4 . MgAl2 O4 films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al2 O3 crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate. 相似文献
8.
ALEXANDRA NAVROTSKY BARRY A. WECHSLER KAREN GEISINGER FRIEDRICH SEIFERT 《Journal of the American Ceramic Society》1986,69(5):418-422
Solution calorimetry of MgAl2 O4 -Al8/3 O4 solid solutions was performed in a molten 2PbO · B2 O3 solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al2 O3 and on the MgAl2 O4 -Al8/3 O4 (MgO-Al2 O3 ) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al8/3 O4 in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values. 相似文献
9.
Jafar F. Al-Sharab Frederic Cosandey Amit Singhal Ganesh Skandan James Bentley 《Journal of the American Ceramic Society》2006,89(7):2279-2285
Nanostructured MgAl2 O4 spinel was synthesized by a direct conversion process from cubic γ-Al2 O3 . The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl2 O4 phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl2 O4 spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78. 相似文献
10.
Subsolidus phase relations in the system iron oride-Al2 O2 -Cr2 O3 in air and at 1 atm. O2 pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O2 , pressure the monoclinic phase Fe2 O3 . Al2 O3 with some Cr2 O3 in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O2 pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given. 相似文献
11.
P. L. ROEDER F. P. GLASSER† E. F. OSBORN 《Journal of the American Ceramic Society》1968,51(10):585-593
Liquidus phase equilibrium data are presented for the system Al2 O3 -Cr2 O3 -SiO2 . The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al2 O3 -ICr2 O3 -93SiO2 ) coexists with a mullite solid solution (61Al2 O3 -10Cr2 O3 -29SiO2 ), a corundum solid solution (19Al2 O3 -81Cr2 O3 ), and cristobalite (SO2 ). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases. 相似文献
12.
MgAl2 O4 spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)3 dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl2 O4 spinel formation from >1280° to ∼900°C. 相似文献
13.
Solid-state reactions between Li2 O and Al2 O3 were studied in the region between Li2 O.Al2 O 3 and Al2 O 3 . The compound Li2 O Al2 O 3 melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li2 O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li2 O -5Al2 O3 , was the only other compound observed. The effect of Li2 O on the sintering of alumina was investigated. 相似文献
14.
Yangyang Ji Jingkui Liang Zhan Chen Sishen Xie 《Journal of the American Ceramic Society》1991,74(2):444-446
The phase relations at a temperature below "subsolidus" in the system Al2 O3 –B2 O3 –Nd2 O3 are reported. Specimens were prepared from various compositions of Al2 O3 , B2 O3 , and Nd2 O3 of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl3 (BO3 )4 (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH2 PO4 (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency. 相似文献
15.
Osamu Yamaguchi Daijo Tomihisa Tatsuji Uegaki Kiyoshi Shimizu 《Journal of the American Ceramic Society》1987,70(11):335-C
In the system Ta2 O3 -Al2 O5 solid solutions of metastable δ-Ta2 O5 (hexagonal) are formed up to 50 mol% Al2 O3 from amorphous materials prepared by the simultaneous hydrolysis of tantalum and aluminum alkoxides. The values of the lattice parameters decrease linearly with increasing Al2 O3 , content. The to β-Ta2 O5 (orthorhombic, low-temperature form) transformation occurs at ∼950°C. The solid solution containing 50 mol% Al2 O3 transforms at 1040° to 1100°C to orthorhombic TaAlO4 . Orthorhombic TaAlO4 contains octahedral TaO6 groups in the structure. 相似文献
16.
Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH4 OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO2 crystallites at elevated temperatures was strongly inhibited by Al2 O3 derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al2 O3 , and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO2 is lower than that of the monoclinic form. With increasing A12 O3 content, however, the transformation temperature gradually increased, although the growth of ZrO2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO2 . The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy. 相似文献
17.
Antonio H. de Aza Juan E. Iglesias Pilar Pena Salvador De Aza 《Journal of the American Ceramic Society》2000,83(4):919-927
Solid-state compatibility and melting relationships in the subsystem Al2 O3 —MgAl2 O4 —CaAl4 O7 were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al2 O3 . The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca2 Mg2 Al28 O46 and CaMg2 Al16 O27 ) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl4 O7 , CaAl12 O19 , MgAl2 O4 , Ca2 Mg2 Al28 O46 , and CaMg2 Al16 O27 were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al2 O3 —MgO—CaO. 相似文献
18.
Phase relations and lattice constants in the MgO–Al2 O3 –Ga2 O3 system at 1550°C have been determined experimentally. In a large part of this system, only a nonstoichiometric spinel is stable. Compositions as extreme as 12.5 mol% MgO–20.5 mol% Ga2 O3 –67 mol% Al2 O3 for a homogeneous spinel are possible. In the bordering phase diagrams of MgO–Al2 O3 and MgO–Ga2 O3 , the composition of the spinel is as high as 63 mol% Al2 O3 or Ga2 O3 , respectively. The contributions of all simple ionic exchange reactions on the lattice constant of the spinel have been deduced from X-ray diffractometry data. 相似文献
19.
Tentative phase relations in the binary system BnOa-A12 O3 are presented as a prerequisite to the understanding of the system Li2 O-B2 O3 -Al2 O3 . Two binary compounds, 2A12 O3 .B2 O3 and 9A12 O3 .-2B2 O3 , melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li2 O.A12 O3 .B2 O3 and 2Li2 O. 2AI2 O3 . 3B2 03 . The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Liz O.-5Al2 O3 and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown. 相似文献
20.
Above 755°C, compounds along the spinel join LiFe5 O8 -Li4 Ti5 O12 form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe5 O8 and the disordered spinel phase. At 800° and 900°C, cubic LiFeO2 ( ss ) and monoclinic Liz Ti03 ( ss ) exist on the monoxide join LiFeO2 -Li2 TiO3 . The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li2 O-Fe2 O3 -TiO2 at 800° and 900°C were determined for compositions containing ∼50 mol% Li2 O. 相似文献